1. Field of the Invention
The present invention relates broadly to a composite fluorocarbon resin based friction material having a higher dynamic coefficient of friction and greater resistance to compression. The invention also relates to the method of making the composite material.
2. Background Information
Composite fluorocarbon resin based friction materials have been available for many years and have provided superior performance in many applications. Of particular value have been materials having a polytetrafluoroethylene (PTFE) resin base. The use of PTFE in friction materials leads to smoother engagement of clutches, synchronizers and the like. These materials also exhibit a more stable dynamic coefficient of friction in relation to applied pressure and outstanding wear resistance, as well as chemical resistance and absorption of particles.
Many such composite materials are known utilizing a wide variety of both fibrous and particulate fillers. U.S. Pat. No. 4,593,802 describes several fluorocarbon, resin based compositions which utilize particulate and/or fibrous fillers with preferred compositions comprising primarily PTFE and elemental, particulate carbon. The dynamic coefficient of friction remains more stable throughout its service life, but the values are on the order of 0.08 to 0.13. Hereafter, in this specification, references to coefficients of friction are to be understood to mean dynamic such coefficients unless specifically referred to as static coefficients.
Additional compositions, especially ones with ceramic fiber filled PTFE, are described in U.S. Pat. No. 4,762,216. However, there is no suggestion that these composites would have higher coefficients of friction. U.S. Pat. No. 4,374,211 describes still more composites including ones with aramid fibers and thermosetting resin binders. The specification is indefinite as to the coefficient of friction in that the patent FIG. shows values of from about 0.0045 to 0.0036, while Table III of the patent shows values from about 0.35 to about 0.50. However, either range is unsuited to many applications.
While these and other prior composites have performed well in many applications, other applications have been unable to use PTFE materials because the coefficient of friction has been too low and wear rates have been too high, especially in those applications involving high engagement pressures.